Some internal combustion engines include engine control systems that deactivate cylinders under low load situations. For example, an eight cylinder can be operated using four cylinders to improve fuel economy by reducing pumping losses. This process is generally referred to as displacement on demand or DOD. Operation using all of the engine cylinders is referred to as an activated mode. A deactivated mode refers to operation using less than all of the cylinders of the engine (one or more cylinders not active).
To smoothly transition between the activated and deactivated modes, the internal combustion engine must produce sufficient drive torque with a minimum of disturbances. Otherwise, the transition will not be transparent to the driver. In other words, excess torque will cause engine surge and insufficient torque will cause engine sag, which degrades the driving experience.
Conventional engine control systems transition between the activated and deactivated modes based on engine vacuum, used as a surrogate for reserve torque, which is commonly referred to as vacuum-based moding. Vacuum-based moding can result in undesired cycling between modes at some ambient conditions. Additionally, transition lags from deactivated to activated modes may occur as a result of intake manifold filling delays, which could cause a slight delay in vehicle acceleration.